Crash consistent snapshot

ABSTRACT

A method, system, and computer program product for splitting IOs directed to a first LUN and a second LUN to a journal on a replication site, creating a snapshot of the first LUN, creating a snapshot of the second LUN, and applying the IO in the journal on the replication site to a copy of the snapshot of the first LUN on the replication site and a copy of the snapshot of the second LUN on the replication site to create a consistent snapshot of the first LUN and the second LUN on the replication site.

A portion of the disclosure of this patent document may contain commandformats and other computer language listings, all of which are subjectto copyright protection. The copyright owner has no objection to thefacsimile reproduction by anyone of the patent document or the patentdisclosure, as it appears in the Patent and Trademark Office patent fileor records, but otherwise reserves all copyright rights whatsoever.

TECHNICAL FIELD

This invention relates to data replication.

BACKGROUND

Computer data is vital to today's organizations, and a significant partof protection against disasters is focused on data protection. Assolid-state memory has advanced to the point where cost of memory hasbecome a relatively insignificant factor, organizations can afford tooperate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, forstoring organizational production site data on a periodic basis. Suchsystems suffer from several drawbacks. First, they require a systemshutdown during backup, since the data being backed up cannot be usedduring the backup operation. Second, they limit the points in time towhich the production site can recover. For example, if data is backed upon a daily basis, there may be several hours of lost data in the eventof a disaster. Third, the data recovery process itself takes a longtime.

Another conventional data protection system uses data replication, bycreating a copy of the organization's production site data on asecondary backup storage system, and updating the backup with changes.The backup storage system may be situated in the same physical locationas the production storage system, or in a physically remote location.Data replication systems generally operate either at the applicationlevel, at the file system level, at the hypervisor level or at the datablock level.

Current data protection systems try to provide continuous dataprotection, which enable the organization to roll back to any specifiedpoint in time within a recent history. Continuous data protectionsystems aim to satisfy two conflicting objectives, as best as possible;namely, (i) minimize the down time, in which the organization productionsite data is unavailable, during a recovery, and (ii) enable recovery asclose as possible to any specified point in time within a recenthistory.

Continuous data protection typically uses a technology referred to as“journaling,” whereby a log is kept of changes made to the backupstorage. During a recovery, the journal entries serve as successive“undo” information, enabling rollback of the backup storage to previouspoints in time. Journaling was first implemented in database systems,and was later extended to broader data protection.

One challenge to continuous data protection is the ability of a backupsite to keep pace with the data transactions of a production site,without slowing down the production site. The overhead of journalinginherently requires several data transactions at the backup site foreach data transaction at the production site. As such, when datatransactions occur at a high rate at the production site, the backupsite may not be able to finish backing up one data transaction beforethe next production site data transaction occurs. If the production siteis not forced to slow down, then necessarily a backlog of un-logged datatransactions may build up at the backup site. Without being able tosatisfactorily adapt dynamically to changing data transaction rates, acontinuous data protection system chokes and eventually forces theproduction site to shut down.

SUMMARY

A method, system, and computer program product for splitting IOsdirected to a first LUN and a second LUN to a journal on a replicationsite, creating a snapshot of the first LUN, creating a snapshot of thesecond LUN, and applying the IO in the journal on the replication siteto a copy of the snapshot of the first LUN on the replication site and acopy of the snapshot of the second LUN on the replication site to createa consistent snapshot of the first LUN and the second LUN on thereplication site.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of embodiments disclosed herein may bebetter understood by referring to the following description inconjunction with the accompanying drawings. The drawings are not meantto limit the scope of the claims included herewith. For clarity, notevery element may be labeled in every figure. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments, principles, and concepts. Thus, features and advantages ofthe present disclosure will become more apparent from the followingdetailed description of exemplary embodiments thereof taken inconjunction with the accompanying drawings in which:

FIG. 1 is a simplified illustration of a data protection system, inaccordance with an embodiment of the present disclosure;

FIG. 2 is a simplified illustration of a write transaction for ajournal, in accordance with an embodiment of the present disclosure;

FIG. 3 is a simplified illustration of a data protection system with aconsistency group spanning two arrays, in accordance with an embodimentof the present disclosure;

FIG. 4 is a simplified illustration of a splitting IO from a consistencygroup spanning two arrays, in accordance with an embodiment of thepresent disclosure;

FIG. 5 is a simplified illustration of taking a first snapshot while asplitting IO from a consistency group spanning two arrays, in accordancewith an embodiment of the present disclosure;

FIG. 6 is a simplified illustration of taking a second snapshot while asplitting IO from a consistency group spanning two arrays, in accordancewith an embodiment of the present disclosure;

FIG. 7 is a simplified illustration of determining the differencesbetween two snapshots of a consistency group spanning two arrays, inaccordance with an embodiment of the present disclosure;

FIG. 8 is a simplified illustration of applying IO journal to twosnapshots on a replication site corresponding to a consistency groupspanning two arrays to create consistent snapshots on the replicationsite, in accordance with an embodiment of the present disclosure;

FIG. 9 is a simplified example of a method for recording changes to LUNsduring taking of snapshot, in accordance with an embodiment of thepresent disclosure;

FIG. 10 is a simplified example of a method for applying recordingchanges to LUNs to create consistent snapshots, in accordance with anembodiment of the present disclosure;

FIG. 11 is an example of an embodiment of an apparatus that may utilizethe techniques described herein, in accordance with an embodiment of thepresent disclosure; and

FIG. 12 is an example of a method embodied on a computer readablestorage medium that may utilize the techniques described herein, inaccordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

Typically, it may be problematic to take a snapshot of a consistencygroup when the consistency group spans multiple arrays. Generally, theremay not be a way to ensure that a snapshot taken on a first array iscrash consistent with a snapshot taken on a second array. Usually, thismay occur as the two snapshots may not be synchronized.

In particular embodiments, the current disclosure may enable crashconsistent snapshots to be taken across multiple arrays. In at leastsome embodiments, the storage arrays may be of different types, such asEMC®'s Symmetrix® and EMC's Clariion® storage arrays. In otherembodiments, the current disclosure may use journaling to enable takingsnapshots at the same point of time across multiple storage arrays. Infurther embodiments, a journal of IO written to a consistency groupspanning different arrays may be recorded. In some embodiments, thejournal may be constructed through the use of one or more splitters oneach array to split the IO to a DPA which splits the IO to a journal. Incertain embodiments, snapshots of LUNs in a consistency group may betaken during splitting of IO to a consistency group. In at least someembodiments, differences between a snapshot taken during splitting and aprevious snapshot may be calculated. In certain embodiments, differencesbetween snapshots may be sent to a replication site. In otherembodiments, a replication site may apply the changes to the snapshotsto a replication snapshot. In further embodiments, IO recorded in ajournal may be applied to snapshots on the replication site to create acrash consistent snapshot.

The following may be helpful in understanding the specification andclaims:

BACKUP SITE—may be a facility where replicated production site data isstored; the backup site may be located in a remote site or at the samelocation as the production site; a backup site may be a virtual orphysical site

CLONE—a clone may be a copy or clone of the image or images, drive ordrives of a first location at a second location;

DELTA MARKING STREAM—may mean the tracking of the delta between theproduction and replication site, which may contain the meta data ofchanged locations, the delta marking stream may be kept persistently onthe journal at the production site of the replication, based on thedelta marking data the DPA knows which locations are different betweenthe production and the replica and transfers them to the replica to makeboth sites identical.

DPA—may be Data Protection Appliance a computer or a cluster ofcomputers, or a set of processes that serve as a data protectionappliance, responsible for data protection services including inter aliadata replication of a storage system, and journaling of I/O requestsissued by a host computer to the storage system; The DPA may be aphysical device, a virtual device running, or may be a combination of avirtual and physical device.

RPA—may be replication protection appliance, is another name for DPA. AnRPA may be a virtual DPA or a physical DPA.

HOST—may be at least one computer or networks of computers that runs atleast one data processing application that issues I/O requests to one ormore storage systems; a host is an initiator with a SAN; a host may be avirtual machine

HOST DEVICE—may be an internal interface in a host, to a logical storageunit;

IMAGE—may be a copy of a logical storage unit at a specific point intime;

INITIATOR—may be a node in a SAN that issues I/O requests;

JOURNAL—may be a record of write transactions issued to a storagesystem; used to maintain a duplicate storage system, and to rollback theduplicate storage system to a previous point in time;

LOGICAL UNIT—may be a logical entity provided by a storage system foraccessing data from the storage system;

LUN—may be a logical unit number for identifying a logical unit; mayalso refer to one or more virtual disks or virtual LUNs, which maycorrespond to one or more Virtual Machines. As used herein, LUN and LUmay be used interchangeably to refer to a LU.

Management and deployment tools—may provide the means to deploy, controland manage the RP solution through the virtual environment managementtools

PHYSICAL STORAGE UNIT—may be a physical entity, such as a disk or anarray of disks, for storing data in storage locations that can beaccessed by address;

PRODUCTION SITE—may be a facility where one or more host computers rundata processing applications that write data to a storage system andread data from the storage system; may be a virtual or physical site

SAN—may be a storage area network of nodes that send and receive I/O andother requests, each node in the network being an initiator or a target,or both an initiator and a target;

SOURCE SIDE—may be a transmitter of data within a data replicationworkflow, during normal operation a production site is the source side;and during data recovery a backup site is the source side; may be avirtual or physical site

SNAPSHOT—a Snapshot may refer to differential representations of animage, i.e. the snapshot may have pointers to the original volume, andmay point to log volumes for changed locations. Snapshots may becombined into a snapshot array, which may represent different imagesover a time period.

STORAGE SYSTEM—may be a SAN entity that provides multiple logical unitsfor access by multiple SAN initiators

TARGET—may be a node in a SAN that replies to I/O requests;

TARGET SIDE—may be a receiver of data within a data replicationworkflow; during normal operation a back site is the target side, andduring data recovery a production site is the target side; may be avirtual or physical site

WAN—may be a wide area network that connects local networks and enablesthem to communicate with one another, such as the Internet.

SPLITTER/PROTECTION AGENT: may be an agent running either on aproduction host a switch or a storage array which can intercept IO andsplit them to a DPA and to the storage array, fail IO redirect IO or doany other manipulation to the IO; the splitter or protection agent maybe used in both physical and virtual systems. The splitter may be in theIO stack of a system and may be located in the hypervisor for virtualmachines. May be referred to herein as an Open Replicator Splitter(ORS).

VIRTUAL VOLUME: may be a volume which is exposed to host by avirtualization layer, the virtual volume may be spanned across more thanone site and or volumes

VASA: may be a set of vCenter providers that allow an administrator tomanage storage

Virtualization filter appliance (VFA): may be a layer in the hypervisorthat has the ability intercepts and split IO from a VM being written toa virtual disk. In some embodiments, the VFA may be running on a VM in ahypervisor

This is an out of mechanism that allows storage management over webbased APIs.

VVOL-filter—may be a VM utilizing a specialized Virtual machine, whichmay provide an infrastructure that allows for introducing a “devicedriver” into the virtualized IO stack provided by the Virtual machine

Virtual RPA (vRPA)/Virtual DPA (vDPA): may be an DPA running in a VM.

VASA may be vSphere Storage application program interfaces (APIs) forStorage Awareness.

DISTRIBUTED MIRROR: may be a mirror of a volume across distance, eithermetro or geo, which is accessible at all sites.

BLOCK VIRTUALIZATION: may be a layer, which takes backend storagevolumes and by slicing concatenation and striping create a new set ofvolumes, which serve as base volumes or devices in the virtualizationlayer

MARKING ON SPLITTER: may be a mode in a splitter where intercepted IOsare not split to an appliance and the storage, but changes (meta data)are tracked in a list and/or a bitmap and I/O is immediately sent todown the IO stack.

FAIL ALL MODE: may be a mode of a volume in the splitter where all writeand read IOs intercepted by the splitter are failed to the host, butother SCSI commands like read capacity are served.

GLOBAL FAIL ALL MODE: may be a mode of a volume in the virtual layerwhere all write and read IOs virtual layer are failed to the host, butother SCSI commands like read capacity are served.

LOGGED ACCESS: may be an access method provided by the appliance and thesplitter, in which the appliance rolls the volumes of the consistencygroup to the point in time the user requested and let the host accessthe volumes in a copy on first write base.

VIRTUAL ACCESS: may be an access method provided by the appliance andthe splitter, in which the appliance exposes a virtual volume from aspecific point in time to the host, the data for the virtual volume ispartially stored on the remote copy and partially stored on the journal.

CDP: Continuous Data Protection, may refer to a full replica of a volumeor a set of volumes along with a journal which allows any point in timeaccess, the CDP copy is at the same site, and maybe the same storagearray of the production site

CRR: Continuous Remote Replica may refer to a full replica of a volumeor a set of volumes along with a journal which allows any point in timeaccess at a site remote to the production volume and on a separatestorage array.

A description of journaling and some techniques associated withjournaling may be described in the patent titled METHODS AND APPARATUSFOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION and with U.S.Pat. No. 7,516,287, and METHODS AND APPARATUS FOR OPTIMAL JOURNALING FORCONTINUOUS DATA REPLICATION and with U.S. Pat. No. 8,332,687, which arehereby incorporated by reference. A description of synchronous andasynchronous replication may be described in the patent titledDYNAMICALLY SWITCHING BETWEEN SYNCHRONOUS AND ASYNCHRONOUS REPLICATIONand with U.S. Pat. No. 8,341,115, which is hereby incorporated byreference.

A discussion of image access may be found in U.S. patent applicationSer. No. 12/969,903 entitled “DYNAMIC LUN RESIZING IN A REPLICATIONENVIRONMENT” filed on Dec. 16, 2010 assigned to EMC Corp., which ishereby incorporated by reference.

Description of Embodiments Using of a Five State Journaling Process

Reference is now made to FIG. 1, which is a simplified illustration of adata protection system 100, in accordance with an embodiment of thepresent invention. Shown in FIG. 1 are two sites; Site I, which is aproduction site, on the right, and Site II, which is a backup site, onthe left. Under normal operation the production site is the source sideof system 100, and the backup site is the target side of the system. Thebackup site is responsible for replicating production site data.Additionally, the backup site enables rollback of Site I data to anearlier pointing time, which may be used in the event of data corruptionof a disaster, or alternatively in order to view or to access data froman earlier point in time.

During normal operations, the direction of replicate data flow goes fromsource side to target side. It is possible, however, for a user toreverse the direction of replicate data flow, in which case Site Istarts to behave as a target backup site, and Site II starts to behaveas a source production site. Such change of replication direction isreferred to as a “failover”. A failover may be performed in the event ofa disaster at the production site, or for other reasons. In some dataarchitectures, Site I or Site II behaves as a production site for aportion of stored data, and behaves simultaneously as a backup site foranother portion of stored data. In some data architectures, a portion ofstored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another,or they may both be situated at a common site, local to one another.Local data protection has the advantage of minimizing data lag betweentarget and source, and remote data protection has the advantage is beingrobust in the event that a disaster occurs at the source side.

The source and target sides communicate via a wide area network (WAN)128, although other types of networks are also adaptable for use withthe present invention.

In accordance with an embodiment of the present invention, each side ofsystem 100 includes three major components coupled via a storage areanetwork (SAN); namely, (i) a storage system, (ii) a host computer, and(iii) a data protection appliance (DPA). Specifically with reference toFIG. 1, the source side SAN includes a source host computer 104, asource storage system 108, and a source DPA 112. Similarly, the targetside SAN includes a target host computer 116, a target storage system120, and a target DPA 124.

Generally, a SAN includes one or more devices, referred to as “nodes”. Anode in a SAN may be an “initiator” or a “target”, or both. An initiatornode is a device that is able to initiate requests to one or more otherdevices; and a target node is a device that is able to reply torequests, such as SCSI commands, sent by an initiator node. A SAN mayalso include network switches, such as fiber channel switches. Thecommunication links between each host computer and its correspondingstorage system may be any appropriate medium suitable for data transfer,such as fiber communication channel links.

In an embodiment of the present invention, the host communicates withits corresponding storage system using small computer system interface(SCSI) commands.

System 100 includes source storage system 108 and target storage system120. Each storage system includes physical storage units for storingdata, such as disks or arrays of disks. Typically, storage systems 108and 120 are target nodes. In order to enable initiators to send requeststo storage system 108, storage system 108 exposes one or more logicalunits (LU) to which commands are issued. Thus, storage systems 108 and120 are SAN entities that provide multiple logical units for access bymultiple SAN initiators.

Logical units are a logical entity provided by a storage system, foraccessing data stored in the storage system. A logical unit isidentified by a unique logical unit number (LUN). In an embodiment ofthe present invention, storage system 108 exposes a logical unit 136,designated as LU A, and storage system 120 exposes a logical unit 156,designated as LU B.

In an embodiment of the present invention, LU B is used for replicatingLU A. As such, LU B is generated as a copy of LU A. In one embodiment,LU B is configured so that its size is identical to the size of LU A.Thus for LU A, storage system 120 serves as a backup for source sidestorage system 108. Alternatively, as mentioned hereinabove, somelogical units of storage system 120 may be used to back up logical unitsof storage system 108, and other logical units of storage system 120 maybe used for other purposes. Moreover, in certain embodiments of thepresent invention, there is symmetric replication whereby some logicalunits of storage system 108 are used for replicating logical units ofstorage system 120, and other logical units of storage system 120 areused for replicating other logical units of storage system 108.

System 100 includes a source side host computer 104 and a target sidehost computer 116. A host computer may be one computer, or a pluralityof computers, or a network of distributed computers, each computer mayinclude inter alia a conventional CPU, volatile and non-volatile memory,a data bus, an I/O interface, a display interface and a networkinterface. Generally a host computer runs at least one data processingapplication, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host devicefor each logical unit exposed by a storage system in the host computerSAN. A host device is a logical entity in a host computer, through whicha host computer may access a logical unit. In an embodiment of thepresent invention, host device 104 identifies LU A and generates acorresponding host device 140, designated as Device A, through which itcan access LU A. Similarly, host computer 116 identifies LU B andgenerates a corresponding device 160, designated as Device B.

In an embodiment of the present invention, in the course of continuousoperation, host computer 104 is a SAN initiator that issues I/O requests(write/read operations) through host device 140 to LU A using, forexample, SCSI commands. Such requests are generally transmitted to LU Awith an address that includes a specific device identifier, an offsetwithin the device, and a data size. Offsets are generally aligned to 512byte blocks. The average size of a write operation issued by hostcomputer 104 may be, for example, 10 kilobytes (KB); i.e., 20 blocks.For an I/O rate of 50 megabytes (MB) per second, this corresponds toapproximately 5,000 write transactions per second.

System 100 includes two data protection appliances, a source side DPA112 and a target side DPA 124. A DPA performs various data protectionservices, such as data replication of a storage system, and journalingof I/O requests issued by a host computer to source side storage systemdata. As explained in detail hereinbelow, when acting as a target sideDPA, a DPA may also enable rollback of data to an earlier point in time,and processing of rolled back data at the target site. Each DPA 112 and124 is a computer that includes inter alia one or more conventional CPUsand internal memory.

For additional safety precaution, each DPA is a cluster of suchcomputers. Use of a cluster ensures that if a DPA computer is down, thenthe DPA functionality switches over to another computer. The DPAcomputers within a DPA cluster communicate with one another using atleast one communication link suitable for data transfer via fiberchannel or IP based protocols, or such other transfer protocol. Onecomputer from the DPA cluster serves as the DPA leader. The DPA clusterleader coordinates between the computers in the cluster, and may alsoperform other tasks that require coordination between the computers,such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 arestandalone devices integrated within a SAN. Alternatively, each of DPA112 and DPA 124 may be integrated into storage system 108 and storagesystem 120, respectively, or integrated into host computer 104 and hostcomputer 116, respectively. Both DPAs communicate with their respectivehost computers through communication lines such as fiber channels using,for example, SCSI commands.

In accordance with an embodiment of the present invention, DPAs 112 and124 are configured to act as initiators in the SAN; i.e., they can issueI/O requests using, for example, SCSI commands, to access logical unitson their respective storage systems. DPA 112 and DPA 124 are alsoconfigured with the necessary functionality to act as targets; i.e., toreply to I/O requests, such as SCSI commands, issued by other initiatorsin the SAN, including inter alia their respective host computers 104 and116. Being target nodes, DPA 112 and DPA 124 may dynamically expose orremove one or more logical units.

As described hereinabove, Site I and Site II may each behavesimultaneously as a production site and a backup site for differentlogical units. As such, DPA 112 and DPA 124 may each behave as a sourceDPA for some logical units and as a target DPA for other logical units,at the same time.

In accordance with an embodiment of the present invention, host computer104 and host computer 116 include protection agents 144 and 164,respectively. Protection agents 144 and 164 intercept SCSI commandsissued by their respective host computers, via host devices to logicalunits that are accessible to the host computers. In accordance with anembodiment of the present invention, a data protection agent may act onan intercepted SCSI commands issued to a logical unit, in one of thefollowing ways:

-   -   Send the SCSI commands to its intended logical unit.    -   Redirect the SCSI command to another logical unit.    -   Split the SCSI command by sending it first to the respective        DPA. After the DPA returns an acknowledgement, send the SCSI        command to its intended logical unit.    -   Fail a SCSI command by returning an error return code.    -   Delay a SCSI command by not returning an acknowledgement to the        respective host computer.

A protection agent may handle different SCSI commands, differently,according to the type of the command. For example, a SCSI commandinquiring about the size of a certain logical unit may be sent directlyto that logical unit, while a SCSI write command may be split and sentfirst to a DPA associated with the agent. A protection agent may alsochange its behavior for handling SCSI commands, for example as a resultof an instruction received from the DPA.

Specifically, the behavior of a protection agent for a certain hostdevice generally corresponds to the behavior of its associated DPA withrespect to the logical unit of the host device. When a DPA behaves as asource site DPA for a certain logical unit, then during normal course ofoperation, the associated protection agent splits I/O requests issued bya host computer to the host device corresponding to that logical unit.Similarly, when a DPA behaves as a target device for a certain logicalunit, then during normal course of operation, the associated protectionagent fails I/O requests issued by host computer to the host devicecorresponding to that logical unit.

Communication between protection agents and their respective DPAs mayuse any protocol suitable for data transfer within a SAN, such as fiberchannel, or SCSI over fiber channel. The communication may be direct, orvia a logical unit exposed by the DPA. In an embodiment of the presentinvention, protection agents communicate with their respective DPAs bysending SCSI commands over fiber channel.

In an embodiment of the present invention, protection agents 144 and 164are drivers located in their respective host computers 104 and 116.Alternatively, a protection agent may also be located in a fiber channelswitch, or in any other device situated in a data path between a hostcomputer and a storage system.

What follows is a detailed description of system behavior under normalproduction mode, and under recovery mode.

In accordance with an embodiment of the present invention, in productionmode DPA 112 acts as a source site DPA for LU A. Thus, protection agent144 is configured to act as a source side protection agent; i.e., as asplitter for host device A. Specifically, protection agent 144replicates SCSI I/O requests. A replicated SCSI I/O request is sent toDPA 112. After receiving an acknowledgement from DPA 124, protectionagent 144 then sends the SCSI I/O request to LU A. Only after receivinga second acknowledgement from storage system 108 may host computer 104initiate another I/O request.

When DPA 112 receives a replicated SCSI write request from dataprotection agent 144, DPA 112 transmits certain I/O informationcharacterizing the write request, packaged as a “write transaction”,over WAN 128 to DPA 124 on the target side, for journaling and forincorporation within target storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety ofmodes of transmission, including inter alia (i) a synchronous mode, (ii)an asynchronous mode, and (iii) a snapshot mode. In synchronous mode,DPA 112 sends each write transaction to DPA 124, receives back anacknowledgement from DPA 124, and in turns sends an acknowledgement backto protection agent 144. Protection agent 144 waits until receipt ofsuch acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protectionagent 144 upon receipt of each I/O request, before receiving anacknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combinesthem into an aggregate “snapshot” of all write activity performed in themultiple I/O requests, and sends the snapshot to DPA 124, for journalingand for incorporation in target storage system 120. In snapshot mode DPA112 also sends an acknowledgement to protection agent 144 upon receiptof each I/O request, before receiving an acknowledgement back from DPA124.

For the sake of clarity, the ensuing discussion assumes that informationis transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A fromDPA 112, and performs journaling and writing to storage system 120. Whenapplying write operations to storage system 120, DPA 124 acts as aninitiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in thejournal, so as to restore storage system 120 to the state it was at, atan earlier time.

As described hereinabove, in accordance with an embodiment of thepresent invention, LU B is used as a backup of LU A. As such, duringnormal production mode, while data written to LU A by host computer 104is replicated from LU A to LU B, host computer 116 should not be sendingI/O requests to LU B. To prevent such I/O requests from being sent,protection agent 164 acts as a target site protection agent for hostDevice B and fails I/O requests sent from host computer 116 to LU Bthrough host Device B.

In accordance with an embodiment of the present invention, targetstorage system 120 exposes a logical unit 176, referred to as a “journalLU”, for maintaining a history of write transactions made to LU B,referred to as a “journal”. Alternatively, journal LU 176 may be stripedover several logical units, or may reside within all of or a portion ofanother logical unit. DPA 124 includes a journal processor 180 formanaging the journal.

Journal processor 180 functions generally to manage the journal entriesof LU B. Specifically, journal processor 180 (i) enters writetransactions received by DPA 124 from DPA 112 into the journal, bywriting them into the journal LU, (ii) applies the journal transactionsto LU B, and (iii) updates the journal entries in the journal LU withundo information and removes already-applied transactions from thejournal. As described below, with reference to FIGS. 2 and 3A-3D,journal entries include four streams, two of which are written whenwrite transaction are entered into the journal, and two of which arewritten when write transaction are applied and removed from the journal.

Reference is now made to FIG. 2, which is a simplified illustration of awrite transaction 200 for a journal, in accordance with an embodiment ofthe present invention. The journal may be used to provide an adaptor foraccess to storage 120 at the state it was in at any specified point intime. Since the journal contains the “undo” information necessary torollback storage system 120, data that was stored in specific memorylocations at the specified point in time may be obtained by undoingwrite transactions that occurred subsequent to such point in time.

Write transaction 200 generally includes the following fields:

-   -   one or more identifiers;    -   a time stamp, which is the date & time at which the transaction        was received by source side DPA 112;    -   a write size, which is the size of the data block;    -   a location in journal LU 176 where the data is entered;    -   a location in LU B where the data is to be written; and    -   the data itself.

Write transaction 200 is transmitted from source side DPA 112 to targetside DPA 124. As shown in FIG. 2, DPA 124 records the write transaction200 in four streams. A first stream, referred to as a DO stream,includes new data for writing in LU B. A second stream, referred to asan DO METADATA stream, includes metadata for the write transaction, suchas an identifier, a date & time, a write size, a beginning address in LUB for writing the new data in, and a pointer to the offset in the dostream where the corresponding data is located. Similarly, a thirdstream, referred to as an UNDO stream, includes old data that wasoverwritten in LU B; and a fourth stream, referred to as an UNDOMETADATA, include an identifier, a date & time, a write size, abeginning address in LU B where data was to be overwritten, and apointer to the offset in the undo stream where the corresponding olddata is located.

In practice each of the four streams holds a plurality of writetransaction data. As write transactions are received dynamically bytarget DPA 124, they are recorded at the end of the DO stream and theend of the DO METADATA stream, prior to committing the transaction.During transaction application, when the various write transactions areapplied to LU B, prior to writing the new DO data into addresses withinthe storage system, the older data currently located in such addressesis recorded into the UNDO stream.

By recording old data, a journal entry can be used to “undo” a writetransaction. To undo a transaction, old data is read from the UNDOstream in a reverse order, from the most recent data to the oldest data,for writing into addresses within LU B. Prior to writing the UNDO datainto these addresses, the newer data residing in such addresses isrecorded in the DO stream.

The journal LU is partitioned into segments with a pre-defined size,such as 1 MB segments, with each segment identified by a counter. Thecollection of such segments forms a segment pool for the four journalingstreams described hereinabove. Each such stream is structured as anordered list of segments, into which the stream data is written, andincludes two pointers—a beginning pointer that points to the firstsegment in the list and an end pointer that points to the last segmentin the list.

According to a write direction for each stream, write transaction datais appended to the stream either at the end, for a forward direction, orat the beginning, for a backward direction. As each write transaction isreceived by DPA 124, its size is checked to determine if it can fitwithin available segments. If not, then one or more segments are chosenfrom the segment pool and appended to the stream's ordered list ofsegments.

Thereafter the DO data is written into the DO stream, and the pointer tothe appropriate first or last segment is updated. Freeing of segments inthe ordered list is performed by simply changing the beginning or theend pointer. Freed segments are returned to the segment pool for re-use.

A journal may be made of any number of streams including less than ormore than 5 streams. Often, based on the speed of the journaling andwhether the back-up is synchronous or a synchronous a fewer or greaternumber of streams may be used.

Consistent Snapshots

Refer now to the example embodiment of FIG. 3. In this exampleembodiment Host 300 sends IOs 310 and 320 to storage array 330 andstorage array 340. IOs 310 and 320 are directed to LUNs 335 and 345which are part of consistency group 352. Storage array 330 has asnapshot 390 at time T_old and storage array 340 has snapshot 395 attime T_old. Replication site 355 has replica LUN 365 of LUN 335 andreplica 366 of LUN 345 which has data identical to snap 390 and snap395. In certain embodiments, the data from the production site may havebeen transferred to the replica site by a snapshot shipping mechanism.In certain embodiments, snapshots on a production site, such assnapshots 390 and 395, may not be consistent with one another.Replication site also has DO stream 360. Replication appliance 350 isalso illustrated in FIG. 3. The embodiment of FIG. 3 may be configuredto create replica point in time copies of the production LUNs which arecrash consistent between the two arrays.

Refer now to the example embodiments of FIGS. 4 and 9. In FIG. 4,storage array 430 has splitter 470. Storage array 440 has splitter 475.To create a snapshot consistent across arrays 330, 340, Splitter 470 isconfigured to intercept IO 410 from host 400 before IO 410 is written toLUN 435. Splitter 470 splits IO 410 to RPA 450 (step 900). Splitter 475is configured to split IO 420 to RPA 450 (step 900). RPA 450 sends thesplit IO to DO stream 460 on replication site 455 (step 910). In mostembodiments, splitting IO may be making a copy of the IO before the IOis passed to a LUN for that IO.

Refer now to the example embodiments of FIGS. 5 and 9. In theseembodiments, storage array 530 takes snapshot 580 of LUN 535 at time T1(steps 910, 915). IOs 510 and 520 are being split by splitters 570 and575, respectively, to RPA 550, which is recording the IO in DO stream560 on replication site 555.

Refer now to the example embodiments of FIGS. 6 and 9. In theseembodiments, storage array 640 takes snapshot 685 of LUN 645 at time T2(steps 910, 915). IOs 610 and 620 are being split by splitters 670 and675, respectively, to RPA 650, which is recording the IO in DO stream660 on replication site 655. Splitters 670 and 675 stop splitting IO(step 920).

Refer now to the example embodiments of FIGS. 7 and 10. In these exampleembodiments, the differences between snapshot 780 at time T1 andsnapshot 790 at Told are determined (step 1005). The differences betweensnapshot 785 at time T2 and snapshot 795 at Told are determined (step1005). Delta snapshot changes 787 from storage array 730 and deltasnapshot changes 789 from storage array 740 are sent to replication site755 (step 1010). The Delta snapshots are applied to replica 765 at Toldand replica volume 766 at Told (step 1015). In most embodiments applyingthe changes to the snapshot at the replication site may make the data atthe replication site equivalent to the snapshot on the storage arrays. Asnapshot is created (1017) of replica LU 765 as SNAP 767 and replica LU766 as SNAP 768.

Refer now to the example embodiments of FIGS. 8 and 10. In the exampleembodiments of FIGS. 8 and 10, IO in DO Stream 860 is applied to Snap867 and Snap 868 at replica site to bring Snap 866 to be Snap 868 attime Tconsistent and Snap 868 to be at time Tconsistent. In mostembodiments, applying the IO tracked in the DO journal and the changesbetween the old snapshot and the snapshot taken while the system wasjournaling IO changes, may result in a crash consistent snapshot at thereplication site.

The methods and apparatus of this invention may take the form, at leastpartially, of program code (i.e., instructions) embodied in tangiblemedia, such as floppy diskettes, CD-ROMs, hard drives, random access orread only-memory, or any other machine-readable storage medium. When theprogram code is loaded into and executed by a machine, such as thecomputer of FIG. 11, the machine becomes an apparatus for practicing theinvention. When implemented on one or more general-purpose processors,the program code combines with such a processor 1103 to provide a uniqueapparatus that operates analogously to specific logic circuits. As sucha general purpose digital machine can be transformed into a specialpurpose digital machine. FIG. 12 shows Program Logic 1234 embodied on acomputer-readable medium 1230 as shown, and wherein the Logic is encodedin computer-executable code configured for carrying out the reservationservice process of this invention and thereby forming a Computer ProgramProduct 1200. The logic 1234 may be the same logic 1140 on memory 1104loaded on processor 1103. The program logic may also be embodied insoftware modules, as modules, or as hardware modules.

The logic for carrying out the method may be embodied as part of thesystem described below, which is useful for carrying out a methoddescribed with reference to embodiments shown in, for example, FIG. 9and FIG. 10. For purposes of illustrating the present invention, theinvention is described as embodied in a specific configuration and usingspecial logical arrangements, but one skilled in the art will appreciatethat the device is not limited to the specific configuration but ratheronly by the claims included with this specification.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present implementations are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A system for data replication, the systemcomprising: a production site including: a first storage array with afirst Logical Unit (LUN) and a first splitter; a second storage arraywith a second LUN and a second splitter; a consistency group; whereinthe consistency group spans the first storage array and the secondstorage array by including the first LUN and the second LUN; areplication site with a journal; and computer-executable logic operatingin memory, wherein the computer-executable program logic is configuredfor execution of: splitting Input/Outputs (IOs) directed to the firstLUN and the second LUN to the journal on the replication site; creatinga snapshot of the first LUN; creating a snapshot of the second LUN;wherein the second snapshot is taken without ensuring the snapshot ofthe first LUN is consistent with the snapshot of the second LUN;applying at least some of the IO in the journal on the replication siteto a copy of the snapshot of the first LUN on the replication site and acopy of the snapshot of the second LUN on the replication site to createa first consistent snapshot of the first LUN and create a secondconsistent snapshot of the second LUN on the replication site; whereinthe first consistent snapshot and the second consistent snapshot areconsistent with respect to the first array and the second array and atthe same point in time, wherein the first and second consistent snapshoton the replication appliance are crash consistent snapshots captured ata consistent point in time; wherein the splitting of the IO is stoppedafter the snapshots of the first LUN and the snapshot of the second LUNare created at the production site.
 2. The system of claim 1 wherein theprogram logic is further configured for: determining a first set ofdifferences between the snapshot of the first LUN and a first oldsnapshot on the first array; determining a second set of differencesbetween the snapshot of the first LUN and a second old snapshot on thesecond array.
 3. The system of claim 2 wherein the program logic isfurther configured for: sending the snapshot of the first LUN as a firstset of differences between the snapshot of the first LUN and the firstold snapshot to the replication site; and sending the snapshot of thesecond LUN as a second set of differences between the snapshot of thesecond LUN and the second old snapshot to the replication site.
 4. Thesystem of claim 3 wherein the program logic is further configured for:applying the first set of differences to the copy of the snapshot of thefirst LUN on the replication site; and applying the second set ofdifferences to the copy of the snapshot of the first LUN on thereplication site on the replication site.
 5. A computer implementedmethod for taking snapshots in data replication comprising: splittingInput/Outputs (IOs) directed to a first Logical Unit (LUN) on a firststorage array and a second LUN on a second storage array to a journal ona replication site; creating a snapshot of the first LUN; creating asnapshot of the second LUN; wherein the second snapshot is taken withoutensuring the snapshot of the first LUN is consistent with the snapshotof the second LUN; applying at least some of the IO in the journal onthe replication site to a copy of the snapshot of the first LUN on thereplication site and a copy of the snapshot of the second LUN on thereplication site to create a first consistent snapshot of the first LUNand create a second consistent snapshot of the second LUN on thereplication site; wherein the first consistent snapshot and the secondconsistent snapshot are consistent with respect to the first array andthe second array and at the same point in time, wherein the first andsecond consistent snapshot on the replication appliance are crashconsistent snapshots captured at a consistent point in time, wherein thesplitting of the IO is stopped after the snapshots of the first LUN andthe snapshot of the second LUN are created at a production site.
 6. Themethod of claim 5 further comprising: determining a first set ofdifferences between the snapshot of the first LUN and a first oldsnapshot on the first array; determining a second set of differencesbetween the snapshot of the first LUN and a second old snapshot on thesecond array.
 7. The method of claim 6 further comprising sending thesnapshot of the first LUN as a first set of differences between thesnapshot of the first LUN and the first old snapshot to the replicationsite; and sending the snapshot of the second LUN as a second set ofdifferences between the snapshot of the second LUN and the second oldsnapshot to the replication site.
 8. The method of claim 7 furthercomprising: applying the first set of differences to the copy of thesnapshot of the first LUN first replication snapshot; and applying thesecond set of differences to the copy of the snapshot of the second LUNsecond replication snapshot on the replication site.
 9. A computerprogram product comprising: a non-transitory computer readable mediumencoded with computer executable program, the code enabling: splittingInput/Outputs (IOs) directed to a first Logical Unit (LUN) on a firststorage array and a second LUN on a second storage array to a journal ona replication site; creating a snapshot of the first LUN; sending a copyof the snapshot of the first LUN to the replication site; creating asnapshot of the second LUN; wherein the second snapshot is taken withoutensuring the snapshot of the first LUN is consistent with the snapshotof the second LUN; sending a copy of the snapshot of the first LUN tothe replication site; applying at least some of the IO in the journal onthe replication site to a copy of the snapshot of the first LUN on thereplication site and a copy of the snapshot of the second LUN on thereplication site to create a first consistent snapshot of the first LUNand create a second consistent snapshot of the second LUN on thereplication site; wherein the first consistent snapshot and the secondconsistent snapshot are consistent with respect to the first array andthe second array and at the same point in time, wherein the first andsecond consistent snapshot on the replication appliance are crashconsistent snapshots captured at a consistent point in time; andstopping the splitting after the snapshots of the first LUN and thesnapshot of the second LUN are created at a production site.
 10. Thecomputer program product of claim 9 wherein code further enables:determining a first set of differences between the snapshot of the firstLUN and a first old snapshot on the first array; determining a secondset of differences between the snapshot of the first LUN and a secondold snapshot on the second array.
 11. The computer program product ofclaim 10 wherein code further enables: sending the snapshot of the firstLUN as a first set of differences between the snapshot of the first LUNand the first old snapshot to the replication site; and sending thesnapshot of the second LUN as a second set of differences between thesnapshot of the second LUN and the second old snapshot to thereplication site.
 12. The computer program product of claim 11 whereincode further enables: applying the first set of differences to the copyof the snapshot of the first LUN first replication snapshot; andapplying the second set of differences to the copy of the snapshot ofthe second LUN second replication snapshot on the replication site. 13.The computer program product of claim 9 wherein the split IO is sent toa data protection appliance; wherein the data protection appliance sendsthe split IO to the journal on the replication site.
 14. The system ofclaim 1 wherein the split IO is sent to a data protection appliance;wherein the data protection appliance sends the split IO to the journalon the replication site.
 15. The computer implemented method of claim 5wherein the split IO is sent to a data protection appliance; wherein thedata protection appliance sends the split IO to the journal on thereplication site.